This invention relates to conductive, soft urethane rollers, e.g., developer rollers, charge rollers, or toner transfer rollers, for use in an electrophotographic printer, e.g., a laser printer. The invention also relates to an electrophotographic printer containing such a roller.
A laser printer often includes a digital light emitter photoconductor drum, a charge roller, a developer roller, a developer blade, a transfer roller, and a toner storage unit. During printing, the transfer roller supplies toner to the developer roller, and the developer blade forms the toner into a thin, even layer on the surface of the developer roller. The charge roller, which is in contact with the surface of the photoconductive drum, receives a high voltage from a power supply, and charges the photoconductive drum with a positive or negative charge. After the photoconductive drum has been exposed a light emitter, the surface of the photoconductor drum forms an electrostatic latent image. The voltage difference between the developer roll and the photoconductive drum allows the former to transfer toner to the drum surface. Areas of the drum surface that are discharged by the laser attract the toner, whereas areas that are more highly charged repel the toner. The discharged and charged areas thus form toner images. The laser printer may also include a toner-removal wiper that removes excess toner from the surface of the developer roller after the developer roller has contacted the photoconductor drum. The toner on the drum subsequently is transferred to paper, and then fuses to form the print.
Many different designs of laser printers are known. They include Shiraki et al., U.S. Pat. No. 5,768,668; Sato, U.S. Pat. No. 5,752,146; Park, U.S. Pat. No. 5,727,022; Okada et al., U.S. Pat. Nos. 5,669,047 and 5,655,197; Sakaguchi, U.S. Pat. No. 5,602,631; Iguchi et al., U.S. Pat. No. 5,600,417; Ikeda et al., U.S. Pat. No. 5,367,367; Kogo et al., U.S. Pat. Nos. 5,324,885 and 5,214,239; Kinoshita et al., U.S. Pat. Nos. 5,311,264 and 5,287,150; Nishio, U.S. Pat. Nos. 5,241,343, 5,076,201, and 5,062,385; and Goyert et al., U.S. Pat. No. 4,521,582; all of which are incorporated herein by reference.
The charge roller and developer roller are usually cylindrical in shape and typically include a central shaft surrounded by a synthetic rubber or urethane elastomer portion. Many charge rollers for use in electrophotographic printing contain multiple layers of rubber coating or foam coating with carbon black filler inside. Due to the need to coat the surface with multiple layers, low production and high cost result.
Coating a charge roller of high conductivity and soft durometer with one layer of rubber/foam (specifically, urethane) has been a challenging task. Most soft charge rollers having high conductivity (typically, 5E5-5E7) need to contain plasticizer to maintain the right hardness (typically, 25 Shore A-50 Shore A). The use of plasticizer, however, may lead to tackiness on the surface of the roller, which can result in poor print quality.
Having the right hardness is also important for proper functioning of a developer roller. Typically, a developer roller has a durometer ranging from about 50 Shore A to 65 Shore A. For use in a high speed, high resolution printer, rollers are typically required to have lower durometer (e.g., 30-50 Shore A). When the hardness drops to below 50 Shore A, the surface of the roller can become too tacky for the toner powder to be properly transferred from the developer roller to the drum, resulting in poor print quality. On the other hand, a hardness of more than 50 Shore A may lead to partial melting of the toner for high speed, high resolution printers.
In general, the invention features a conductive roller (e.g., a developer roller or charge roller) for use in laser printers or other electrophotography printers such as photocopier or thermal printer. The roller includes an inner shaft surrounded by a single layer of conductive solid or foam thermoset copolymer formed of reactive silicone and urethane precursors. A roller containing solid thermoset copolymer means that the thermoset urethane portion contains no void. In contrast, a roller containing foam thermoset copolymer contains void, e.g., a microcellular roller. The rollers generally have a uniform volume resistivity ranging from about 1E5 to about 5E10 ohm-cm; preferably, from about 5E5 to about 5E9 ohm-cm, and a hardness of between about 25 Shore A and about 50 Shore A; preferably, between about 30 Shore A and about 45 Shore A.
The thermoset copolymer exhibits a stable volume resistivity even when the humidity changes from about 10% to about 90% and the temperature changes from about 10xc2x0 C. to about 40xc2x0 C. By xe2x80x9cstable volume resistivity xe2x80x9d is meant that the ratio of volume resistivity at 10xc2x0 C., 20% relative humidity (LL) to the volume resistivity at 40xc2x0 C., 90% relative humidity (HH) is less than 50. The thermoset copolymer also exhibits low tackiness under the same conditions (i.e., from LL to HH). The entire copolymer displays uniform conductivity. A roller has low tackiness if it does not adhere to another identical roller during storage, i.e., no pressure is applied to the rollers, and the coefficient of friction is less than 2.0.
In one aspect, the conductive roller contains a metal salt evenly or homogeneously distributed in a copolymer formed of the precursors: (i) a diisocyanate, (ii) a reactive silicone containing an amine group, and (iii) a polyol or a polyamine. The incorporation of silicone into the backbone of the copolymer generally allows the resulting copolymer to have low tackiness property and better resistance to change in humidity and temperature. The roller preferably is substantially free of plasticizer and non-reactive silicone oil. As used herein, a copolymer that is xe2x80x9csubstantially free xe2x80x9d of a material means that the amount of the material is negligible in the composition, i.e., less than about 0.001 wt % of the total weight of the copolymer. By xe2x80x9cnon-reactive xe2x80x9d is meant the silicone oil does not incorporate into the backbone of the copolymer.
In another aspect, the conductive roller has a volume resisitivity of between about 1E5 ohm-cm and about 5E10 ohm-cm and contains a metal salt completely dissolved and evenly or homogeneously distributed in a copolymer formed of the precursors: (i) a diisocyanate, (ii) a reactive silicone containing a hydroxyl group or an amine group, and (iii) a polyol or a polyamine.
The invention also features methods of preparing the roller including the thermoset copolymer, as well as laser printers including the roller.
Other features and advantages will be apparent from the description of the preferred embodiments thereof, and also from the claims.
A preferred conductive roller contains a shaft surrounded by a single layer of solid conductive silicone-containing thermoset copolymer. The outer surface of the roller may be uncoated, or may be coated with, e.g., a rubber such as a urethane rubber, nitrile rubber or silicone rubber. The thickness of the coating may be, e.g., between 1.5 and 10 mil. The shaft can be made of steel, aluminum, a conductive plastic, pultrusion conductive rod, or any other material commonly used for the shaft of a conductive roller.
The conductive roller can be used in photographic printing or electrophotographic printing; preferably, the roller is used in electrophotographic printers, e.g., laser printers, thermal printers, and photocopiers.
In one embodiment, the preferred silicone-containing thermoset copolymer is formed of precursors such as a diisocyanate, a polyol or a polyamine, and a reactive silicone containing an amine group. In another embodiment, the preferred silicone-containing thermoset copolymer, which has a volume resistivity of between about 5E5 ohm-cm and about 5E9 ohm-cm, is formed of precursors such as a diisocyanate, a polyol or a polyamine, and a reactive silicone containing a hydroxyl group and/or an amine group.
The silicone-containing thermoset copolymer also includes a solid solution of metal salt evenly or homogeneously distributed therein, and may further include a catalyst, a light stabilizer, or antioxidant. The copolymer is substantially free of a non-reactive silicone oil or a plasticizer that can migrate to the surface of roller after cure.
The preferred isocyanate is a one-shot or prepolymer materials that have an NCO no less than 2, preferably, about 6-32. NCO is defined as the combined % wt of N, C, and O in 100 g of isocyanate. Specific examples of such an isocyanate include methylene diisocyanate (MDI) (e.g., Isonate 2143L, Isonate 180, Isonate 181, Isonate 191, Isonate 226, Isonate 240, and Isonate 125 M, all available from Dow Chemical, Midland, Mich.; Mondur P. F, Mondur M, Mondur XP-744, Mondur CD, and Mondur E-501, all available from Bayer, Pittsburgh, PA.; Lupranate M10, Lupranate M20S, Lupranate M70L, Lupranate M200, Lupranate No. 78 Iso, Lupranate M, Lupranate MS, Lupranate MP-102, Lupranate 103, and Lupranate 218 Iso, all available from BASF, Parsippany, N.J.); hydrogenated methylene diisocyanate (HMDI) (Uniroyal Chemical, Middlebury, CT); toluene diisocyanate (TDI) (Uniroyal Chemical, Middlebury, CT); p-phenylene diisocyanate (PPDI), (Uniroyal Chemical, Middlebury, CT.); hexamethylene diisocyanate (HDI), (Bayer, Pittsburgh, Pa.); and naphthyl diisocyanate (NDI), (Bayer, Pittsburgh, Pa.). The preferred isocyanate also includes prepolymers cured with esters such as adipate or caprolactone or prepolymers cured with polyether such as polypropylene glycol (PPG), polytetramethylene ethylene glycol (PTMEG), or other types of glycol (Du Pont, Wilmington, Del.). The preferred copolymer can include more than one type of isocyanate, e.g., a combination of MDI and PPDI. The preferred weight percent of the isocyanate ranges from 5 wt % to about 85 wt %, and more preferably, from about 10 wt % to about 84 wt %, based on the total weight of the copolymer.
Examples of the preferred polyol or polyamine include butanediol (XB) (available from GAF Chemicals, Wayne N.J.); tremethanol propane (TMP) (available from Hoechst Celanese, Dallas, Tex.); trisopropylamine (TIPA) (available from Dow Chemical, Midland, Mich.); Isonol 93 (available from Upjohn Co., Kalamazo, Mich.); hydroquinone ethyl ether (HQEE) (available from Eastman Chemical Co., Kingsport, Tenn.); hexanediol (available from Aldrich Chemical, Milwaukee, Wis.); Pluracol Polyol 994LV, Pluracol Polyol 816, Pluracol Polyol 945, Pluracol Polyol 1117, Pluracol Polyol 380, Pluracol Polyol HP-6500T, Pluracol Polyol 538, Pluracol Polyol 220, Pluracol Polyol 628, and Pluracol Polyol TPE 4542 (all available from BASF, Parsippany, N.J.); Acclaim Polyol 4220 and Acclaim Polyol 3000 (available from Arco Chemicals, New Square Pa.); and Polamine 3000 and Polamine 4000 (available from Air Products, Allentown, Pa.). The preferred weight percent of polyol or polyamine ranges from 50 wt % to about 85 wt %, and more preferably, from about 55 wt % to about 84 wt %, based on the total weight of the copolymer. The preferred polyol or polyamine has a molecular weight ranging from about 44 to 15,000; preferably, from about 90 to 10,000.
The reactive silicone precursor contains at least one, preferably, two or more, reactive hydrogen atom. A reactive hydrogen atom refers to a hydrogen atom that is displaced in a coupling reaction, for example, a hydrogen atom of a hydroxyl group or an amine group. The hydroxyl group or the amine group can be present at any position of the reactive silicone precursor. The copolymer can be formed of more than one type of reactive silicone precursor, for example, a combination of a reactive silicone precursor having end-capping hydroxyl groups and a reactive silicone precursor having end-capping amino groups.
A urethane linkage (xe2x80x94NHxe2x80x94C(xe2x95x90O)xe2x80x94Oxe2x80x94) results when a hydroxyl group of the reactive silicone precursor reacts with the diisocyanate; whereas a urea linkage (xe2x80x94NHxe2x80x94C(xe2x95x90O)xe2x80x94NHxe2x80x94) results when an amine group of the reactive silicone precursor reacts with the diisocyanate. In general, copolymers containing polyurea displays higher rigidity than those containing polyurethane. The rigidity of the copolymer can be adjusted by the ratio of the number of hydroxyl groups to the number of amino groups present in the silicone precursor. Moreover, as mentioned above, the preferred silicone-containing copolymers have better resistance towards changes in humidity.
Preferably, the reactive hydrogen atom forms part of an end-capping group of the precursor. An end-capping group, e.g., a hydroxyl or an amine group, is a group attaching to the silicon atom at the ends of the precursor. The preferred reactive silicone precursor can contain an end-capping amino group, an end-capping hydroxyl group, or a mixture of both, thus resulting in a linear copolymer when polymerizes with the diisocyanate. See the general formula below: 
Each of R1, R5, and R8, independently, is hydrogen, alkyl, aryl, aralkyl, alkoxy, aryloxy, aralkoxy, hydroxyalkyl, aminoalkyl, hydroxyalkoxy, aminoalkoxy, hydroxyl, or amino. Preferably, each of R1, R5, and R8, independently, is hydrogen, methyl, ethyl, amino-substituted phenyl, hydroxyl-substituted phenyl, methoxy, ethoxy, hydroxyl, or amino. Each of R2, R3, R4, R6, and R7, independently, is hydrogen, alkyl, aryl, aralkyl, alkoxy, aryloxy, or aralkoxy. Preferably, each of R2, R3, R4, R6, and R7, independently, is hydrogen, methyl, ethyl, phenyl, naphthyl, or alkylphenyl. x is 10-2,000, preferably, 20-500. At least one of R1, R5, and R8 has to contain a hydroxyl group or an amino group. Note that R1, R2, R3, R6, R7, and R8 are end-capping groups of the above-depicted reactive silicone precursor.
Alkyl, as used herein, is a straight or branched hydrocarbon chain containing 1 to 8 carbon atoms. Examples of alkyl include, but are not limited to, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, and 2-methylhexyl. As used herein, aryl is an aromatic group containing 6-12 ring atoms and can contain fused rings, which may be saturated, unsaturated, or aromatic. Examples of an aryl group include phenyl, naphthyl, biphenyl, phenanthryl, and anthracyl.
Examples of the preferred reactive silicone precursor include GP-145, GP-6, and GP7100 (available from Genesee Polymer Corp., Mich.). The preferred weight percent of reactive silicone precursor ranges from 0.5 wt % to about 20 wt %, and more preferably, from about 2 wt % to about 10 wt %, based on the total weight of the copolymer. The preferred reactive silicone precursor has a molecular weight ranging from about 500 to 30,000; preferably, from about 1,000 to 15,000.
The metal salt provides the silicone-containing thermoset copolymer with the appropriate conductivity, for example, a volume resistivity ranges from about 1E5 to about 5E100 ohm-cm, preferably, from about 1E6 to about 5E8 ohm-cm. The metal salt is fully dissolved and evenly or homogeneously complexed with the thermoset copolymer, resulting in a uniform, three-dimensional charge distribution. Examples of metal salts that can be used include transition metal halide salts such as iron chloride, copper chloride, iron bromide, iron iodide, copper iodide, and copper bromide; and lithium salts such as lithium chloride, lithium bromide, and lithium perchlorate. All of these are available from Aldrich Chemical, Milwaukee, Wis. The thermoset copolymer typically includes between about 0.001% and about 5%, preferably, between about 0.002% and 2%, of the metal salt by weight.
In one embodiment, the preferred copolymer, which is formed of a reactive silicone precursor having an amino group, contains one or more of a transition metal salt (e.g., iron chloride or copper chloride). This copolymer can be used in forming a roller in a xerographic printer. In another embodiment, the preferred copolymer, which is formed of a reactive silicone precursor having a hydroxyl group, contains one or more of a lithium salt (e.g., lithium perchlorate). This copolymer can be used in forming a roller in a laser printer.
Examples of catalysts that can be used in forming the copolymer include Fomrez UL-32 and Fomrez 29 (available from Witco, Taft, La.); and Dabco T-12, Dabco T-9, and Dabco 331v (available from Air Products, Allentown, Pa.). The copolymers may include, for example, between about 0.005% and about 1% of the catalyst by weight.
Examples of UV light stabilizers that can be used include Tinuvin P, Tinuvin C 353 FF, Tinuvin 111 FB, Tinuvin 111 FDL, Tinuvin 123, Tinuvin 144, Tinuvin 213, Tinuvin 234, Tinuvin 326, Tinuvin 327, Tinuvin 328, Tinuvin 622LD, Tinuvin 765, Tinuvin 770 DF, Tinuvin 783FB, Tinuvin 783FD, Tinuvin 783FDL, Uvitex OB, Chimassorb 81, Chimassorb 119FL, and Chimassorb 944 LD/FL (all available from Ciba, Tarrytown, N.Y.). Typically, the thermoset copolymer includes between about 0.1% and about 5%; preferably, between about 1% and about 3%, of the stabilizer by weight.
Examples of antioxidants include Irganox 245, Irganox 1010, Irganox 1076, Irganox 1098, Irganox 1135, Irganox 5057, and Naugard (2,6-di-tert-butyl-p-cresol, BHT) (all available from Ciba, Torryton, N.Y.). The thermoset copolymer generally includes between about 0.1% and about 5%; preferably, about 0.1% and about 3%, of the antioxidant by weight.
Both the UV light stabilizer and the antioxidant inhibit the addition reaction and oxidation reaction of the roller surface.
The roller can be prepared by combining the appropriate urethane precursors and other ingredients in a tube or shaft mold that includes a pre-baked shaft with an adhesive coating. The shaft can be, for example, in a rod-shaped or a circular tube, while the mold can be in U-shaped, rectangular, square, or circular. The molding procedure can be, for example, vertical or horizontal casting, spin casting, a centrifugal method, or an extrusion or protrusion process. The mixture is cured for 15-30 minutes at 160-180xc2x0 F., and the resulting roller is demolded and post-cured for 12-16 hours at 180xc2x0 F. in the oven. The desired top coating (if any) is applied to the roller by an extrusion or spray-coating process, and then may undergo finish grinding to provide a final roller with specified dimensioning, resistivity, and surface roughness for use in a laser printer.
In a preferred procedure, the metal salt is at least partially dissolved in polyol/polyamine before being combined with the additional polyol/polyamine, reactive silicone precursor, and isocyanate to form a solution in which the salt is completely dissolved. This typically is accomplished, for example, by heating a mixture including the salt and polyol/polyamine at an elevated temperature (e.g., 80xc2x0 F.-130xc2x0 F.) for one or two hours under vacuum. For purposes of this application, the resultant solution will be referred to as the xe2x80x9cconductive curative. xe2x80x9d The conductive curative also may include, for example, the UV light stabilizer and/or antioxidant and/or coloration.
Conductivity measurement is obtained using the method ASTM D257-78 (Keithley 487 picoammeter/voltage source). Hardness measurement is determined by using Shore instrument.